Since the Brillouin frequency shift (BFS) in Brillouin scattering in optical fibers is affected by the sound velocity, as well as by the fiber refractive index, it has been used to detect variations in strain and temperature in a wide variety of applications. In typical practical applications, optical fibers are secured to or embedded in components whose properties are to be monitored such that variations in the properties of the components result in similar or corresponding variations in the properties of the fibers.
A method of using Brillouin scattering to determine strain and/or temperature variations in a distributed manner along the fiber typically involves injecting a pulsed pump signal into one end of the fiber and a constant wave (CW) probe signal into the other. This is often referred to as stimulated Brillouin scattering. The pump signal will usually be at a fixed frequency and the frequency of the probe signal will be scanned through a suitable range to derive the Brillouin Gain Spectrum “BGS” (or vice versa). The spectrum obtained under test conditions can be compared to the spectrum obtained during steady state conditions. A change in strain or temperature along the fiber will result in a frequency shift of the local BGS which can be measured, and after suitable calibration used to obtain measurements of variations in temperature and/or strain or other BFS-affecting phenomenon. This use of Brillouin scattering to determine variations in properties of materials is sometimes referred to as Brillouin sensing.
Practical applications of Brillouin scattering, using optical fibers to monitor strain, involve lengths of fiber of hundreds of meters to a many tens of kilometers.
Distributed sensing methods based on the Brillouin non-linear scattering effect have recently shown their growing potential for long distance (˜1001 cm) and high spatial resolution (cm's). As noted above, most forms of sensing apparatus are based on the classical Brillouin Optical Time Domain Analysis (BOTDA), which maps the Brillouin Gain spectrum (BGS) along the fiber, from which the strain/temperature-dependent Brillouin frequency Shift (BFS) is determined Several methods have been developed in order to carry the Brillouin distributed sensing to the dynamic domain.
As a result of such developments the accuracy obtainable from measurements of Brillouin scattering is such that vibrations in a fiber caused by applied acoustic signals can be detected.